Innovative procedure for measurement of losses of transformers supplying nonsinusoidal loads

The measurement of iron-core and copper losses of single-phase transformers is important in particular for transformers feeding nonlinear loads. A new digital data-acquisition method is devised for the separate measurement of iron-core and copper losses of transformers under any full or partial-load condition. The accuracy requirements of the instruments (voltage, current sensors, volt and current meters) employed are addressed. A 25 kVA, 7200/240 V single-phase transformer has been loaded with either three diode-bridge rectifiers and/or a thyristor-bridge rectifier at varying total harmonic voltage and current distortions, such that at the maximum permissible apparent power, losses are identical to the rated losses. The maximum errors of losses are acceptably low if voltage (dividers) and current (shunts) sensors with 2% and digital instruments with 0.1% errors are used. The derating under nonlinear loads is measured as a function of dominant 3rd and 5th current harmonics. The influence of voltage distortion is studied. The total harmonic distortion of the current (THD_i) was varied greatly from 0 to 90%, and the resulting voltage THD_v values were below 6% for the given power system impedance. At large THD_i values (e.g. 90%), the reduction in apparent power rating (RAPR) is not very large (≅10%). However, the real power capability (RPC) of the transformer is severely restricted: for example, at THD_i = 90% the RPC is about 0.5 pu. For this reason it might be advisable to define derating in terms of the reduction in real power capability and not in terms of RAPR.